Method and apparatus for measuring oil effluent flow rates

ABSTRACT

The invention relates to a flow rate measurement method adapted to oil effluents made up of multiphase fluid mixtures comprising water, oil, and gas. The effluent is passed through a Venturi in which the effluent is subjected to a pressure drop (Δp), a mean value (&lt;Δp&gt;) of the pressure drop is determined over a period t 1  corresponding to a frequency f 1  that is low relative to the frequency at which gas and liquid alternate in a slug flow regime, a mean value (&lt;ρ m &gt;) is determined for the density of the fluid mixture at the constriction of the Venturi over said period t 1 , and a total mass flow rate value &lt;Q&gt; is deduced for the period t 1  under consideration from the mean values of pressure drop and of density.

The invention relates in general to measurements intended to determineat least one characteristic of oil well effluents made up of multiphasefluids, typically comprising three phases: two liquid phases—crude oiland water—and one gas phase based on hydrocarbons. The characteristicsin question are specifically the proportions of the component phases,including the water content of the liquid phase, and the flow ratevalues—total flow rate and the flow rates of the various phases.

The ability of the oil industry to optimize production of a reservoirrelies on the possibility of evaluating the well effluent at regularintervals, in terms of quantity (flow rate) and of composition (theproportions of the various phases). This makes it possible to determinewhat corrective action may need to be taken. However, measuring the flowrate of oil well effluent is a problem that is complex because of theway effluents are usually made up of three phases, and because of thechanges in flow conditions to which they are subject (pressure,temperature, shape of pipes). These factors give rise to a wide varietyof flow regimes being observed, including some regimes of highlynon-uniform and unstable character, with the proportions of the phasesin the fluid mixture being capable of varying very considerably both inthe flow direction (i.e. over time) and across the flow direction, inthe form of phase stratification across the flow section. One extreme,but very common, case is slug flow i.e. that of a high gas content withthe flow being made up of an alternation of portions that areessentially gas, known as “pockets”, and portions that are constitutedessentially by liquid, known as “plugs”.

In the oil industry, the traditional practice is to separate theeffluent into its component phases and to perform measurements on thephases separated in this way. However that technique requires separatorsto be installed on site, where separators are bulky and expensive itemsof equipment, and while wells are being tested, it also requiresadditional pipes to be put into place.

Numerous proposals have been put forward for developing techniques thatwould make it possible to avoid using such separators. A description ofthese developments is to be found in SPE publication 28515 (SPE AnnualTechnical Conference, New Orleans, Sep. 25-28, 1994) by J. Williars,“Status of multiphase flow measurement research”.

Most of the propositions suggest firstly a total flow rate sensor andsecondly sensors for measuring the proportions of the phases in themixture.

Amongst those proposals, U.S. Pat. No. 4,788,852 describes apparatuscomprising a Venturi and a device for measuring gamma ray attenuation atthree different energy levels, the device being situated at theconstriction of the Venturi.

British patent application 2 128 756 explains that it is necessary todetermine two of the following three magnitudes: total mass flow rate,total volume flow rate, and mean density of the fluid. In order tocompensate for non-uniformities in the flow, which give rise inparticular to differences of speed between the gas and the liquid phase,thereby making any flow rate measurement difficult, and also makingdensity measurements inaccurate, it proposes homogenizing the fluidupstream from the sensors by means of an appropriate device. In additionto improving the quality of mean density measurement, homogenization hasthe effect of equalizing the speeds of the gas and liquid phases, andthus of enabling the gas flow rate to be measured.

An application of that principle is described in document WO 90/13859which provides firstly a Venturi and a gamma ray density meter placed atthe constriction of the Venturi, and secondly a mixer upstream from theVenturi for the purpose of homogenizing the multiphase fluid enteringthe Venturi. Nevertheless, the cost and the size of such apparatus maylimit its commercial applications.

The invention seeks to characterize three-phase oil effluents by meansthat are simple and cheap, and that are applicable to a wide variety offlow regimes, and in particular to slug flow.

In one aspect, the invention provides a flow rate measurement methodadapted to oil effluents made up of multiphase fluid mixtures comprisingwater, oil, and gas, the method comprising the following steps: theeffluent is passed through a Venturi in which the effluent is subjectedto a pressure drop; a mean value of the pressure drop is determined overa period t₁ corresponding to a frequency f₁ that is low relative to thefrequency at which gas and liquid alternate in a slug flow regime; amean value is determined for the density of the fluid mixture at theconstriction of the Venturi over said period t₁; and a total mass flowrate value <Q> is deduced for the period t₁ under consideration from themean values of pressure drop and of density.

Appropriately, the density of the fluid mixture is measured by gamma rayattenuation at a first energy level at a frequency f₂ that is highrelative to said frequency of gas/liquid alternation in a slug flowregime, and the mean of the measurements obtained in this way over eachperiod t₁ corresponding to the frequency f₁ is formed to obtain saidmean density value.

Appropriately, the frequency f₁ is 0.1 Hz or less than 0.1 Hz, e.g. 0.01Hz. Appropriately, the frequency f₂ is greater than 20 Hz, andpreferably greater than 40 Hz, e.g. being equal to 45 Hz.

The invention will be well understood on reading the followingdescription made with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of flow measuring apparatus suitable for oil welleffluents; and

FIG. 2 is a block diagram illustrating the data processing operationsperformed by the apparatus of FIG. 1, in two embodiments.

Oil effluents are usually made up of a multiphase mixture of liquid oil,of gas (hydrocarbons), and of water. Below we use the followingnotations: the symbols Q and q designate mass flow rates and volume flowrates respectively; the symbol ρ designates density; the symbols α and γdesignate the static and dynamic proportions of the various phases; andthe indices o, w, g, and l refer respectively to the oil, water, gas,and liquid phases (where the liquid phase is the oil and the water takentogether), while the index m designates the fluid mixture.

The device comprises a pipe section 10 comprising a convergent Venturi11 whose narrowest portion 12 is referred to as the throat. In theexample shown, the section of the pipe 10 is disposed vertically and theeffluent flows upwards, as symbolized by arrow F.

The constriction of the flow section in the Venturi induces a pressuredrop Δp between level 13, situated upstream from the Venturi at theinlet to the measurement section, and the throat 12. This pressure dropis associated with the total mass flow rate Q and with the density ρ_(m)by the following equation:${\Delta \quad p} = {\frac{K \cdot Q^{2}}{\rho_{m}} + {\rho_{m} \cdot g \cdot h_{v}}}$

where g is the acceleration due to gravity, h_(v) is the distancebetween the upstream level 13 and the throat 12, and K is a constantassociated essentially with the geometry of the Venturi, and which isgiven by: $K = \frac{1 - \beta^{4}}{2{C^{2} \cdot A^{2}}}$

where β is the constriction ratio of the Venturi, i.e. the ratio betweenthe diameter of the throat and the upstream diameter of the Venturi, Cis the discharge coefficient, and A is the section of the throat. Theterm ρ_(m).g.h_(v) is generally small or negligible. By writingΔp*=Δp−ρ_(m).g.h_(v) , equation (1) becomes:

Q=K(Δp*.ρ_(m))^(½)  (2)

where k=K^(½).

In a preferred embodiment, the ratio ρ is 0.5. With a pipe having adiameter of 10 cm, the diameter of the throat is 5 cm. The dischargecoefficient C is about 1. This coefficient depends to a small extent andin predictable manner on the properties of the fluid. Traditionally,this corrective effect is taken into account by the Reynolds number.

The pressure drop Δp is measured by means of a differential pressuresensor 15 connected to two pressure takeoffs 16 and 17 opening out intothe measurement section respectively at the upstream level 13 and in thethroat 12 of the Venturi. In a variant, the measurement may also beperformed by means of two absolute pressure sensors connected to thepressure takeoffs 16 and 17, respectively.

The density ρ_(m) of the fluid mixture is determined by means of asensor which measures the attenuation of gamma rays, by using a source20 and a detector 21 placed on opposite sides of the Venturi throat 12.The throat is provided with “windows” of a material that shows lowabsorption of photons at the energies under consideration. The source 20produces gamma rays at two different energy levels, referred to below asthe “high energy” level and as the “low energy” level. The detector 21which comprises in conventional manner a scintillator crystal such asNaI and a photomultiplier produces two series of signals W_(hi) andW_(lo) referred to as count rates, representative of the numbers ofphotons detected per sampling period in the energy ranges bracketing theabove-mentioned levels respectively.

These energy levels are such that the high energy count rate W_(hi) isessentially sensitive to the density ρ_(m) of the fluid mixture, whilethe low energy count rate W_(lo) is also sensitive to the compositionthereof, thus making it possible to determine the water content of theliquid phase.

Preferably the high energy level lies in a range 85 keV to 150 keV. Forcharacterizing oil effluent, this energy range presents the remarkableproperty that the mass attenuation coefficient of gamma rays therein issubstantially the same for water, for sodium chloride, and for oil,being about 0.17 cm²/g. This means that based on the high energyattenuation, it is possible to determine the density ρ_(m) of the fluidmixture without the need to perform auxiliary measurements to determinethe properties of the individual phases of the fluid mixture(attenuation coefficients and densities). The attenuation measured bythe detector 21 is expressed by the following equation:

A=D _(v).ν_(m).β_(m)  (3)

where D_(V) is the distance travelled through the fluid, i.e. in thiscase the diameter of the Venturi throat, and ν_(m) is the massattenuation coefficient of the fluid mixture.

Since the mass attenuation coefficients of water and oil in theabove-indicated energy range are substantially identical, and since thecontribution of the gas is negligible because of its very low density,the mass attenuation coefficient ν_(m), and thus the product D_(V).ν_(m)that appears in equation (3) can be considered as being substantiallyconstant and independent of the densities ρ_(o) and ρ_(w) of the oil andwater phases.

Under such conditions, the high energy attenuation A_(hi) is a veryadvantageous indicator of the density ρ_(m) of the mixture.

A material that is suitable for producing high energy gamma rays in theenergy range under consideration, and low energy rays is gadolinium 153.This radioisotope has an emission line at an energy that isapproximately 100 keV (in fact there are two lines around 100 keV, butthey are so close together they can be treated as a single line), andthat is entirely suitable for use as the high energy source. Gadolinium153 also has an emission line at about 40 keV, which is suitable for thelow energy level that is used to determine water content. This levelprovides good contrast between water and oil, since the attenuationcoefficients at this level are significantly different, typical valuesbeing 0.228 cm²/g for oil and 0.291 cm²/g for sea water.

FIG. 1 also shows a pressure sensor 22 connected to a pressure takeoff23 opening out into the throat 12 of the Venturi, which sensor producessignals representative of the pressure p_(v) in the throat of theVenturi, and a temperature sensor 24 producing signals T representativeof the temperature of the fluid mixture. The data p_(v) and T is used inparticular for determining gas density ρ_(g) under the flow rateconditions and gas flow rate q_(g) under normal conditions of pressureand temperature on the basis of the value for the flow rate under theflow rate conditions, determined in a manner described below. In thisrespect, it is preferable for the pressure to be measured at the throatof the Venturi. In contrast, it does not matter where temperature ismeasured. The information coming from the above-mentioned sensors isapplied to a data processing unit 30 constituted by a computer running aprogram for delivering the looked-for results by performing varioustreatments based on the principle explained below. In the explanation,reference is made to the block diagram of FIG. 2.

1—Determining Total Mass Flow Rate

The need is to determine the total mass flow rate Q in a manner that isadapted to the particularly difficult case of the effluent flow regimebeing of the pocket-and-plug type, also called slug flow i.e. when theflow is in the form of alternating portions made up essentially of gas,known as “pockets”, and portions made up essentially of liquid.

A mass flow rate value <Q>, referred to below as the mean mass flowrate, is determined (block 35) by applying above equation (2) to valuesdefined as means <Δp*> and <ρ_(m)> of the pressure drop Δp and of thedensity of the mixture ρ_(m) over a time interval t₁ which is longrelative to the gas/liquid alternation period of a pocket-and-plug flow:

<Q>=k(<Δp*><ρ _(m)>)^(½)  (4)

It has been found that this method gives results that are robust and ofsatisfactory accuracy, contrary to what might have been expected, giventhat equation (2) is non-linear and equation (4) is a priori onlyapplicable to a single-phase liquid since the values both of pressuredrop Δp and of density ρ_(m) are subject to extremely sudden variationsin a slug flow made up of pockets and plugs. Such a method ofdetermining mass flow rate transforms a multiphase flow, even a flowthat is extremely non-uniform such as slug flow, into a virtualsingle-phase flow, in that it makes equation (4) above, that applies tosingle-phase flows, applicable to multiphase flows. The method can thusbe depicted as performing a “virtual homogenization”.

The period of gas/liquid alternations in a slug flow can vary widely,depending on circumstances: typically it lies in the range 0.1 to 10seconds. The duration t₁ over which the above-mentioned mean values aredefined must be long relative to said alternation period. It isappropriate to set it at a value that is not less than the upper limitof the above-mentioned range, i.e. 10 s, which corresponds to afrequency, written f₁, of 0.1 Hz.

In the preferred implementation of the invention, these mean values areobtained as follows.

As explained above, the density is determined from the high energyattenuation W_(hi) at a frequency f₂ which is high relative to thegas/liquid alternation frequency in a slug flow (block 40). Startingfrom values ρ_(m)(i) respectively obtained for intervals t₂corresponding to the frequency f₂, a mean density value <ρ_(m)> iscalculated over a duration corresponding to above-mentioned frequency f₁(block 41). Such a method is adapted to the non-linear character of theequation relating the count rate W_(hi) to the density ρ_(m) of thefluid mixture. It is appropriate for the frequency f₂ to be at least 20Hz, preferably at least 40 Hz, and for example 45 Hz.

Measurements are acquired at such a sampling frequency by means of aspecialized electronic circuit associated with the detector 21 but notshown in FIG. 1. The circuit is implemented in accordance with theteaching of U.S. Pat. No. 5,349,195, and it need not be described indetail herein. Such a circuit provides fast and reliable processing ofthe pulses sensed in each energy window.

The inherent response time of the differential pressure sensor 15 isconsiderably slower than the time corresponding to the frequency f₂, forexample it is about 0.5 Hz. However, to simplify implementation,differential pressure measurement samples are similarly acquired at thefrequency f₂ (45 Hz in the example described) and the desired mean value<Δp> is obtained by averaging the measured samples Δp(i). The mean value<Δp*> used in determining the total mass flow in application of aboveequation (4) is obtained from the mean values <Δp> and <ρm>.

2 —Determining Water Content

The detector 21 measures attenuation of gamma rays by the fluid mixtureat the two above-mentioned energy levels:

W _(hi) =W _(hi,0)exp(−A _(hi,m))

W _(lo) =W _(lo,0)exp(−A _(lo,m))

where A stands for attenuation and W_(hi,0) and W_(lo,0) are the highand low count rates in the absence of attenuation, as obtained bycalibration.

As already mentioned above, the attenuation of the gamma rays by thefluid mixture is as follows:

A_(m)=D_(v).ν_(m).ρ_(m)

where D_(v) is the diameter of the throat of the Venturi and ν_(m) isthe mass attenuation coefficient of the fluid mixture.

Given that the density of the gas phase is very low, it is possible tomake the following approximations:

ν_(m)≈ν_(l)

and

ρ_(m)≈α_(l).ρ_(l)

where ν_(l) is the mass attenuation coefficient of the liquid (oil andwater together) and α_(l) is the volume fraction of the liquid, andhence:

A_(m)≈D_(v).ν_(l).ρ_(l).α_(l)

The mass attenuation coefficient of the liquid ν_(l) is a function ofthe attenuation coefficients of oil and water ν_(o) and ν_(w), and ofthe water/liquid mass ratio (WLR):

ν_(l)=ν_(i)+WLR(ν_(w)−ν_(o))

The water/liquid mass ratio WLR can be determined from the high and lowenergy attenuations in the mixture, given the attenuations in oil and inwater as determined by calibration, by using the following expression:

WLR=(λ_(m)−λ_(o))/(λ_(w)−λ_(o))  (5)

where λ_(m), λ_(o), and λ_(w) are the ratios of high energy attenuationover low energy attenuation for the fluid mixture, for oil, and forwater, respectively:

λm=A_(hi,m)/A_(lo,m)

λo=A_(hi,o)/A_(lo,o)

λw=A_(hi,w)/A_(lo,w)

The values of the water/liquid mass ratio WLR(i) are thus calculated foreach of the sampling intervals t₂(i) corresponding to the frequency f₂which is equal to 45 Hz in the example described, on the basis of themeasurements W_(hi)(i) and W_(lo)(i) acquired over the interval underconsideration (block 50). Thereafter (block 51) a mean value <WLR> iscalculated over the interval t₁ corresponding to the frequency f₁ atwhich the mass flow rate Q is determined, with the values WLR(i) beingweighted by respective confidence coefficients C(i).

The water ratio WLR is relative to the quantity of liquid in the fluidmixture. The measurement of WLR therefore applies to this quantity ofliquid. When the gas content is high, the liquid content is small. Alsoattenuation is low because the density of the fluid mixture is low.Under such circumstances, the measurements relating to the proportionsof water and oil in the liquid cannot be of good quality. The confidencecoefficient should therefore decrease with increasing gas content.

Since the density ρ_(m) of the fluid mixture is a sensitive indicator ofits gas content, the confidence coefficient C(i) is advantageouslydetermined as a function of the density ρ_(m). (This choice is justifiedby the assumption that there is no systematic correlation between thecomposition of the liquid as expressed by the water fraction WLR, andthe gas content as given by the density ρ_(m) of the mixture.) Such afunction can have several different forms. By way of example, theconfidence coefficient may merely be equal to the density ρ_(m).Naturally, other types of function can be envisaged. Also, the parameterused could be the variation in the density ρ_(m) rather than the densityproper, or a combination of these factors could be used.

To sum up, each value of WLR(i) is associated with a confidencecoefficient C(i) determined as a function of the density ρ_(m)(i)obtained as described above, and the mean value <WLR> is a weighted meanof the values WLR(i):

 <WLR>=(ΣC(i).WLR(i))/ΣC(i)

where Σ symbolizes the sum of the values lying in the interval t₁ underconsideration. From the mean value <WLR> obtained for the mass fractionof water, it is possible to determine the density of the liquid phaseρ_(l) and the mean value of the volume fraction of water <wlr>, knowingthe densities ρ_(w) and ρ_(o) of water and oil respectively in theeffluent. The density values ρ_(w) and ρ_(o) and the density of gasρ_(g) (under the flow conditions) can be obtained in various ways byauxiliary measurements that are known per se. By way of example, for theliquid phases, mention can be made of the sampling device described inFrench patent application 96 14292 filed on Nov. 22, 1996.

The density of the liquid phase is obtained (block 52) from thefollowing equation:

1/ρ_(l) =<WLR>/ρ _(w)+(1−<WLR>)/ρ_(o)

and the mean volume fraction of water (block 53) is given by:

<wlr>=ρ_(w)/ρ_(l).<WLR>

It should be observed that the water content of the liquid phase is acharacteristic of the effluent which is stable (or at any rate whichvaries slowly), unlike the gas content of the mixture. It is thereforeappropriate to determine ρ_(l) at the frequency f₁, as a function of themean value <WLR>.

In addition, starting from the density of the liquid phase ρl determinedas described above, and from the density of the gas phase ρ_(g) underflow conditions, it is possible (block 54) to determine the proportionsof liquid α_(l) and of gas α_(g) at the frequency f₂ using the followingequations:

α_(l)(i)=(ρ_(m)(i)−ρ_(g))/(ρ_(l)−ρ_(g))

α_(l)+α_(g)=1

3—Determining the Oil and the Water Flow Rates

The gas mass flow rate Q_(g) is negligible relative to the liquid massflow rate because of the low density of the gas. It can therefore beassumed that the liquid mass flow rate Q_(l) is approximately equal tothe total mass flow rate Q:

Q_(l)≈Q

The oil and water mass flow rates can then be determined (block 60)approximately from the following equations:

Q_(w)=<WLR>.<Q>

and

Q_(o)=<Q>−Q_(w)

The water and oil volume flow rates q_(w) and q_(o) are deduced from themass flow rates Q_(w) and Q_(o) (block 61) given the respectivedensities ρ_(w) and ρ_(o) of water and oil in the effluent:

q_(w)=Q_(w)/r_(w)

and

q_(o)=Q_(o)/ρ_(o)

The volume flow rate of the liquid q_(l) can then be determined as thesum of the oil and water flow rates:

q_(l)=q_(o)+q_(w)

or equivalently:

q_(l)=Q_(l)/ρ_(l)≈Q/ρ_(l)

The values obtained in this way are approximate since they assume thatthe gas flow rate is negligible. For more accurate determination,account is taken of the presence of gas, expressed by the volumefraction of the gas. Since the speed of gas is higher than that of theliquid (a phenomenon known as “slip”), it is necessary to make use notof the liquid and gas proportions α_(l) and α_(g) as deduced in themanner given above from the gamma ray attenuation measurements sincethose proportions are static proportions, but instead to make use ofdynamic proportions γ_(l) and γ_(g) (with γ_(l)+γ_(g)=1) which take theslip phenomenon into account, which proportions are respectively lowerthan and greater than the static proportions α_(l) and α_(g). This isexpressed by the equation:

γ_(g)=α_(g)+δ_(g)(α_(g))

where δ_(g)(α_(g)) is the slip term and represents a positivecorrection.

When dealing with values at the throat of the Venturi, it has been foundthat the dynamic proportions can be determined as a function solely ofthe gas proportion α_(g) and are therefore essentially independent offlow rate. A suitable example of the empirical equation making suchdetermination possible is of the form:

γ_(g)(i)=α_(g)(i)+A(γl(i))^(m)(1−γ_(l)(i))^(n)

where the coefficient A and the exponents m and n are determinedempirically. A suitable example corresponds to the following values:A=1, m=1, n=6. This equation makes it possible by iterative computationto determine the dynamic proportion γ_(l) from the static proportionα_(l) (block 55).

Using the values of γ_(l)(i) as determined at the high frequency f₂ (45Hz in this example), a mean value <γ_(l)> is calculated (block 56) overeach interval t₁ (i.e. 10 s in the example described) corresponding tothe frequency f₁.

The liquid volume flow rate q_(l) can then be obtained at frequency f₁from the total mass flow rate <Q>. The total volume flow rate, takingaccount of slip, is given (block 70) by the following equation:

q=Q/ρ_(H)

where the magnitude ρ_(H) is a value for the density of the mixture,known as the “homogeneous density”, that takes account of the slipphenomenon, and that is obtained (block 57) from the equation:

ρ_(H)=<γ_(l)>ρ_(l)+<γ_(g)>ρ_(g)

and the liquid flow rate q_(l) is given (block 71) by:

q_(l)=qγ_(l)

From which the water and oil flow rates are deduced (block 72):

q_(w)=<wlr>q_(l)

q_(o)=(1−<wlr>)q_(l)

4—Determining Gas Flow Rate

To determine the gas flow rate, a suitable method, different from thatdescribed above, relies on an “instantaneous” value for the total massflow rate Q(i) calculated (block 80) for each sample corresponding tothe high frequency f₂ (45 Hz in the example described) in application ofthe following equation:

Q(i)=k[(<Δp>−ρ _(m)(i).g.h_(ν)).ρ_(m)(i)]^(½)

with the notation being the same as above.

An “instantaneous” value for the total volume flow rate is then deduced(block 81):

q(i)=Q(i)/ρ_(H)(i)

where ρ_(H)(i) is the “homogeneous density” as defined above, determinedfor each sample at the high frequency f₂ (block 58) as follows:

 ρ_(H)(i)=γ_(l)(i)ρ_(l)+γ_(g)(i)ρ_(g)

Based on this value for the total volume flow rate, an “instantaneous”value for the gas flow rate q_(g)(i) is determined (block 82) for eachsample at the high frequency f₂:

q _(g)(i)=γ_(g)(i)q(i)

and finally the gas flow rate q_(g) is calculated by averaging thevalues q_(g)(i) over each interval t₁ (10 seconds in the exampledescribed) corresponding to the low frequency f₁ (block 83).

What is claimed is:
 1. A flow rate measurement method adapted to oileffluents made up of multiphase fluid mixtures comprising water, oil,and gas, comprising the following steps: the effluent is passed througha Venturi in which the effluent is subjected to a pressure drop (Δp); amean value (<Δp>) of the pressure drop is determined over a period t₁corresponding to a frequency f₁ that is low relative to the frequency atwhich gas and liquid alternate in a slug flow regime; a mean value(<ρ_(m)>) is determined for the density of the fluid mixture at theconstriction of the Venturi over said period t₁; and a total mass flowrate value <Q> is deduced for the period t₁ under consideration from themean values of pressure drop and of density.
 2. A method according toclaim 1, in which the density of the fluid mixture (ρ_(m)) is measuredby gamma ray attenuation at a first energy level at a frequency f₂ thatis high relative to said frequency of gas/liquid alternation in a slugflow regime, and the mean (<ρ_(m)>) of the measurements ρ_(m)(i)obtained in this way over each period t₁ corresponding to the frequencyf₁ is formed to obtain said mean density value.
 3. A method according toclaim 1, in which the frequency f₁ is not greater than 0.1 Hz.
 4. Amethod according to claim 2, in which the frequency f₂ is not less than20 Hz.
 5. A method according to claim 2, in which the attenuation ofgamma rays by the fluid mixture is measured at the frequency f₂ at asecond energy level lower than the first energy level, this measurementbeing performed at the constriction of the Venturi, and a value WLR(i)is deduced therefrom for the mass ratio of water relative to the liquidcomponent of the fluid mixture.
 6. A method according to claim 5, inwhich each of the measurements at said second energy level is associatedwith a confidence coefficient relating to the corresponding valueρ_(m)(i) of the density of the fluid mixture, said coefficient beingsmall when the density is low and higher at densities corresponding to alow proportion of gas.
 7. A method according to claim 5, in whichapproximate values for the mass flow rate Q_(o) and Q_(w) and for thevolume flow rates q_(o) and q_(w) of the oil and of the water arededuced from said total flow rate value <Q> and the water ratio WLR. 8.A method according to claim 5, in which the static proportions of liquidα_(l)(i) and of gas α_(g)(i) at the frequency f₂ are deduced from themeasurements of the fluid mixture density ρ_(m)(i) and of the water massratio WLR(i) at said frequency.
 9. A method according to claim 8, inwhich the dynamic proportions of liquid γ_(l)(i) and of gas γ_(g)(i) atthe frequency f₂ are deduced from the corresponding static proportionsusing a determined relationship.
 10. A method according to claim 9, inwhich, from said dynamic proportions, mean values <γ_(l)> and <γ_(g)>are determined over each interval t₁ corresponding to said frequency f₁.11. A method according to claim 10, in which the total volume flow rateis deduced from the total mass flow rate <Q>, from said mean values<γ_(l)> and <γ_(g)>, and from the liquid and gas densities, and thevolume flow rates of oil and of water are deduced therefrom.
 12. Amethod according to claim 9, in which total mass flow rate values Q(i)are determined at the frequency f₂ as a function of the correspondingdensity values ρ_(m)(i), gas flow rate values q_(g)(i) are deduced fromsaid values Q(i), from the dynamic proportions of liquid γ_(l)(i) and ofgas γ_(g)(i), and from the liquid and gas densities, and the mean of thevalues q_(g)(i) is calculated to obtain a value for the gas flow rateover each interval t₁.
 13. A flow rate measurement method comprising thefollowing steps: determining a mean value (<Δp>) of the pressure drop ofa fluid mixture over a period t₁ corresponding to a frequency f₁ that islow relative to the frequency at which gas and liquid alternate in aslug flow regime; determining a mean value (<ρ_(m)>) of the density ofthe fluid mixture over said period t₁; and determining a total mass flowrate value <Q> for the period t₁ under consideration from the meanvalues of pressure drop and of density.
 14. The method according toclaim 13, in which a density of the fluid mixture (ρ_(m)(i)) is measuredby gamma ray attenuation at a frequency f₂ that is high relative to saidfrequency f1.
 15. The method according to claim 14, in which the meanvalue (<ρ_(m)>) is determined from ρ_(m)(i) over the period t₁corresponding to the frequency f₁.
 16. The method according to claim 13,in which the frequency f₁ is not greater than 0.1 Hz.
 17. The methodaccording to claim 14, in which the frequency f₂ is not less than 20 Hz.18. The method according to claim 14, wherein said density of the fluidmixture ρ_(m)(i) is measured by gamma ray attenuation at a first energylevel.
 19. The method according to claim 18, wherein said density of thefluid mixture is measured by gamma ray attenuation at a second energylevel lower than the first energy level.
 20. The method according toclaim 19, wherein a value WLR(i) is deduced for the mass ratio of waterrelative to the liquid component of the fluid mixture.
 21. The methodaccording to claim 19, wherein each value WLR(i) is associated aconfidence coefficient relating to the corresponding value ρ_(m)(i) ofthe density of the fluid mixture.
 22. The method according to claim 21,wherein a mean value <WLR> is determined from the values WLR(i).
 23. Themethod of claim 22, wherein said mean value <WLR> is determined over aperiod t₁ corresponding to a frequency f₁ that is low relative to thefrequency at which gas and liquid alternate in a slug flow regime. 24.The method according to claim 23, wherein approximate values for themass flow rate Q_(o) and Q_(w) and for the volume flow rates q_(o) andq_(w) of the oil and of the water are deduced from said total flow ratevalue <Q> and the mean value <WLR>.
 25. The method according to claim20, wherein static proportions of liquid α_(l)(i) and of gas α_(g)(i) atthe frequency f₂ are deduced from the measurements of the fluid mixturedensity ρ_(m)(i) and of WLR(i) at said frequency.
 26. The methodaccording to claim 25, in which dynamic proportions of liquid γ_(l)(i)and of gas γ_(g)(i) at the frequency f₂ are deduced from thecorresponding static proportions.
 27. The method according to claim 26,wherein mean values <γ_(l)> and <γ_(g)> are determined from said dynamicproportions over each interval t₁ corresponding to said frequency f₁.28. The method according to claim 27, wherein the total volume flow rateis deduced from the total mass flow rate <Q>, from said mean values<γ_(l)> and <γ_(g)>, and from the liquid and gas densities, and thevolume flow rates of oil and of water are deduced therefrom.
 29. Themethod according to claim 14, wherein total mass flow rate values Q(i)are determined at the frequency f₂ as a function of the correspondingdensity values ρ_(m)(i).
 30. The method of claim 29, wherein gas flowrate values q_(g)(i) are deduced from said values Q(i), from the dynamicproportions of liquid γ_(l)(i) and of gas γ_(g)(i), and from the liquidand gas densities.
 31. The method of claim 30, wherein a mean a valuefor the gas flow rate qg is determined over each interval t₁ from thevalues q_(g)(i).
 32. The method of claim 13, wherein said fluid mixtureis passed through a Venturi.
 33. An apparatus for flow rate measurementcomprising: a Venturi to receive a fluid mixture; a pressure sensor,coupled to said Venturi, to determine a mean value (<Δp>) of a pressuredrop of said fluid mixture over a period t₁ corresponding to a frequencyf₁ that is low relative to the frequency at which gas and liquidalternate in a slug flow regime; and a data processing unit, coupled tosaid pressure sensor and to said Venturi, to determine a mean value(<ρ_(m)>) of the density of the fluid mixture over said period t₁ and todetermine a total mass flow rate value <Q> for the period t₁ underconsideration from the mean values of pressure drop and of density. 34.The apparatus of claim 33, further including a source of gamma rays anda detector of said gamma rays.
 35. The apparatus of claim 34, whereinsaid source of gamma rays configured to produce gamma rays through saidVenturi and said detector configured to produce a signal sensitive to adensity ρ_(m) of the fluid mixture in response to the gamma raysproduced through said Venturi.
 36. The apparatus of claim 35, whereinsaid detector coupled to said data processing unit to provide saidsignal.
 37. The apparatus of claim 36, in which a density (ρ_(m)(i)) ofthe fluid mixture is measured by gamma ray attenuation at a frequency f₂that is high relative to said frequency f1.
 38. The apparatus of claim37, in which the mean value (<ρ_(m)>) is determined from ρ_(m)(i) overthe period t₁ corresponding to the frequency f₁.
 39. The apparatus ofclaim 33, in which the frequency f₁ is not greater than 0.1 Hz.
 40. Theapparatus of claim 37, in which the frequency f₂ is not less than 20 Hz.41. The apparatus of claim 37, wherein said density of the fluid mixtureρ_(m)(i) is measured by gamma ray attenuation at a first energy level.42. The apparatus of claim 41, wherein said density of the fluid mixtureis measured by gamma ray attenuation at a second energy level lower thanthe first energy level.
 43. The apparatus of claim 42, wherein a valueWLR(i) is deduced for the mass ratio of water relative to the liquidcomponent of the fluid mixture.
 44. The method according to claim 37,wherein each value WLR(i) is associated a confidence coefficientrelating to the corresponding value ρ_(m)(i) of the density of the fluidmixture.
 45. The apparatus of claim 44, wherein a mean value <WLR> isdetermined from the values WLR(i).
 46. The apparatus of claim 45,wherein said mean value <WLR> is determined over a period t₁corresponding to a frequency f₁ that is low relative to the frequency atwhich gas and liquid alternate in a slug flow regime.
 47. The apparatusof claim 46, wherein approximate values for the mass flow rate Q_(o) andQ_(w) and for the volume flow rates q_(o) and q_(w) of the oil and ofthe water are deduced from said total flow rate value <Q> and the meanvalue <WLR>.
 48. The apparatus of claim 47, wherein static proportionsof liquid α_(l)(i) and of gas α_(g)(i) at the frequency f₂ are deducedfrom the measurements of the fluid mixture density ρ_(m)(i) and ofWLR(i) at said frequency.
 49. The apparatus of claim 48, in whichdynamic proportions of liquid γ_(l)(i) and of gas γ_(g)(i) at thefrequency f₂ are deduced from the corresponding static proportions. 50.The apparatus of claim 49, wherein mean values <γ_(l)> and <γ_(g)> aredetermined from said dynamic proportions over each interval t₁corresponding to said frequency f₁.
 51. The apparatus of claim 50,wherein the total volume flow rate is deduced from the total mass flowrate <Q>, from said mean values <γ_(l)> and <γ_(g)>, and from the liquidand gas densities, and the volume flow rates of oil and of water arededuced therefrom.
 52. The apparatus of claim 37, wherein total massflow rate values Q(i) are determined at the frequency f₂ as a functionof the corresponding density values ρ_(m)(i).
 53. The apparatus of claim52, wherein gas flow rate values q_(g)(i) are deduced from said valuesQ(i), from the dynamic proportions of liquid γ_(l)(i) and of gasγ_(g)(i), and from the liquid and gas densities.
 54. The apparatus ofclaim 53, wherein a mean a value for the gas flow rate qg is determinedover each interval t₁ from the values q_(g)(i).